1. Field of the Invention
The present invention relates to methods and apparatus for producing expansion-molded articles.
2. Description of the Background Art
As a process for molding foamed particles of a polyolefin resin, there has heretofore been used only a process in which the foamed particles are filled into a mold and heated with steam to secondarily expand the foamed particles, thereby mutually fusion-bonding them (such a molding process will hereinafter be referred to as "batch type molding process"). On the other hand, in the case of foamed particles of a polystyrene resin, there is also used a process in which the foamed particles are caused to pass through a heating region by holding them between belts provided on upper and lower sides to convey them, thereby continuously molding them (such a process will hereinafter be referred to as "continuous molding process"). The process described in Japanese Patent Publication No. 2424/1977, in which a steam heating method is used, the process described in Japanese Patent Publication No. 1632/1966, in which a high-frequency heating method is used, the process described in Japanese Patent Publication No. 42621/1972, in which a hot-air heating method is used, and the like have been known. The continuous molding process has the advantages of permitting the continuous production of molded articles from foamed particles and the provision of molded articles of continuous length compared with the batch type molding process.
However, such a high-frequency heating method as described in Japanese Patent Publication No. 1632/1966 involves a problem that it requires a large and expensive apparatus and a possibility that sparks may be emitted upon high-frequency heating, and is unsuitable for use as a heating means for foamed particles of a polyolefin resin, which must be heated with a heating temperature range controlled narrow. Such a hot-air heating method as described in Japanese Patent Publication No. 42621/1972 is also hard to control the heating temperature, is difficult to evenly heat the foamed particles because the heat capacity of hot air is smaller than steam, and is hence also unsuitable for use as a heating means for continuously molding the foamed particles of the polyolefin resin.
On the other hand, the steam heating method is a method useful even for the foamed particles of the polyolefin resin. However, an attempt to mold the foamed particles of the polyolefin resin by the continuous molding process using such steam heating as described in Japanese Patent Publication No. 2424/1977 has involved such problems that steam for heating the foamed particles leaks out on the feeding side of the foamed particles to cause failures in fusion bonding among the foamed particles and in secondary expansion of the foamed particles due to insufficient heating, and that when the leakage of the steam becomes greater, the foamed particles fed come to flow backward on the feeding side. The reason for this is considered to be attributable to a difference in secondary expandability between the foamed particles of the polystyrene resin and the foamed particles of the polyolefin resin. More specifically, the foamed particles of the polystyrene resin undergo secondary expansion at a relatively low temperature (generally, 100.degree. C. or lower) because the polystyrene resin is noncrystalline and has good retention of a foaming agent used in the preparation of the foamed particles, and so the foamed particles contain about several percent of the foaming agent. Therefore, when the foamed particles of the polystyrene resin are continuously molded, the fact that the foamed particles gently undergo secondary expansion before they reach a heating region, and so interparticle spaces are closed, and the fact that the foamed particles of the polystyrene resin can be molded with steam of pressure as relatively low as about 1.0 kg/cm.sup.2 G can between them easily prevent the steam from leaking out on the feeding side of the foamed particles. It is consequently considered that reliable molding becomes feasible.
On the other hand, the polyolefin resin is crystalline, and a foaming agent used in the preparation of the foamed particles thereof escapes out of the particles in a relatively short period of time. Therefore, in order to secondarily expand the foamed particles of the polyolefin resin, it is necessary to heat the foamed particles at a temperature higher than the case of the foamed particles of the polystyrene resin. It is thus not easy to secondarily expand the foamed particles of the polyolefin resin to such an extent that the leakage of the steam can be prevented before they reach the heating region. In addition, the secondary expansion of the foamed particles of the polyolefin resin requires to feed high-pressure steam. It is therefore considered that even if the foamed particles of the polyolefin resin could be secondarily expanded to some extent, it was difficult to prevent the leakage of the high-pressure steam by only the secondary expandability of the foamed particles. As described above, the leakage of the heating steam tends to more often occur in the case of the foamed particles of the polyolefin resin, which require steam of pressure higher than the case of the foamed particles of the polystyrene resin in order to provide a satisfactory molded article by secondarily expanding the foamed particles to mutually fusion-bond them. Consequently, such a process has involved a problem that not only the pressure of the heating steam is lowered, thereby failing to sufficiently heat the foamed particles and provide any molded article, but also the foamed particles come to flow backward on the feeding side of the foamed particles when the leakage of the steam becomes greater, resulting in an impossibility of molding.
With the foregoing problems in view, the present applicant proposed, as processes for continuously molding foamed particles of a polyolefin resin, (1) a process in which foamed particles are compressed to a bulk volume of 40-70% on the original bulk volume in a foamed particle-feeding region and then heated with steam (Japanese Patent Application No. 289360/1995), and (2) a process in which foamed particles the internal pressure of which has been raised are gradually compressed in a foamed particle-feeding region and then heated with steam (Japanese Patent Application No. 289361/1995). However, these processes have also still involved the following problems.
Namely, the process (1) has involved a problem that since the foamed particles are heated in the state that they have been compressed to a bulk volume of 40-70% on the original bulk volume, the resulting molded article comes to have an expansion ratio greatly lowered compared with the expansion ratio of the original foamed particles.
On the other hand, according to the process (2), the degree of reduction in expansion ratio of the resulting molded article is smaller than that of the process (1) because the foamed particles the internal pressure of which has been raised is used, and so it is possible to control the reduction in expansion ratio of the molded article compared with the expansion ratio of the original foamed particles to substantially the same degree as that of the molded article obtained by the batch type molding process. However, the process (2) requires to apply an internal pressure considerably higher than the batch type molding process to foamed particles. When the foamed particles to which such a high internal pressure has been applied are used, it takes a longer time to cool the resulting molded article. In the continuous molding process, it therefore involves a problem that a line speed in molding must be slowed down, and so productivity is lowered. Further, in order to apply a high internal pressure to the foamed particles, it is necessary to subject the foamed particles to a pressurizing treatment for a long period of time in a high-pressure tank. This also has formed the factor of reduction in productivity.